U.S. patent application number 10/299225 was filed with the patent office on 2003-06-05 for apparatus and method for symbol combining in a mobile communication system.
This patent application is currently assigned to SAMSUNG ELECTRONICS CO., LTD.. Invention is credited to Ha, Sang-Hyuck, Kim, Min-Goo.
Application Number | 20030103585 10/299225 |
Document ID | / |
Family ID | 19716092 |
Filed Date | 2003-06-05 |
United States Patent
Application |
20030103585 |
Kind Code |
A1 |
Kim, Min-Goo ; et
al. |
June 5, 2003 |
Apparatus and method for symbol combining in a mobile communication
system
Abstract
An apparatus and method for soft-combining demodulated symbols
in a mobile communication system. In the mobile communication
system, a transmitting apparatus modulates symbols in different
modulation schemes and a receiving apparatus demodulates the
modulation symbols in correspondence with the modulation schemes.
The soft symbol combining apparatus includes an RWF calculator, an
RWF controller, and a soft symbol controller. The RWF calculator
calculates an energy of a modulation symbol from each of the
modulation schemes and determines relative ratios of the energies
to be RWFs for the modulation schemes. The RWF controller
multiplies soft metrics of the demodulated soft symbols by the
RWFs. Finally, the soft symbol controller combines the multiplied
soft metrics.
Inventors: |
Kim, Min-Goo; (Suwon-shi,
KR) ; Ha, Sang-Hyuck; (Suwon-shi, KR) |
Correspondence
Address: |
Paul J. Farrell, Esq.
DILWORTH & BARRESE, LLP
333 Earle Ovington Blvd.
Uniondale
NY
11553
US
|
Assignee: |
SAMSUNG ELECTRONICS CO.,
LTD.
Kyungki-do
KR
|
Family ID: |
19716092 |
Appl. No.: |
10/299225 |
Filed: |
November 19, 2002 |
Current U.S.
Class: |
375/340 ;
375/324 |
Current CPC
Class: |
H04L 1/1816 20130101;
H04L 1/0066 20130101; H04L 1/0003 20130101; H04L 1/1819 20130101;
H04L 1/1845 20130101; H04L 1/0045 20130101 |
Class at
Publication: |
375/340 ;
375/324 |
International
Class: |
H03D 001/00; H04L
027/06; H04L 027/14; H04L 027/16; H04L 027/22 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 19, 2001 |
KR |
P2001-71832 |
Claims
What is claimed is:
1. A method of combining demodulated symbols in a mobile
communication system including a transmitting apparatus that
modulates symbols in different modulation schemes and a receiving
apparatus that demodulates the modulation symbols in correspondence
with the different modulation schemes, the method comprising the
steps of: determining weighting factors for the different
modulation schemes; and multiplying the demodulated symbols by the
weighting factors.
2. The method of claim 1, further comprising the step of combining
the multiplied values.
3. The method of claim 1, wherein the weighting factors are
reliability weighting factors (RWFs).
4. The method of claim 1, wherein the weighting factors are a
predetermined values.
5. A method of combining demodulated symbols in a mobile
communication system including a transmitting apparatus that
modulates symbols in different modulation schemes and a receiving
apparatus that demodulates the modulation symbols in correspondence
with the different modulation schemes, the method comprising the
steps of: calculating a log likelihood ratio (LLR) of a modulation
symbol from each of the different modulation schemes; determining
relative ratios of the LLRs to be weighting factors for the
different modulation schemes; multiplying soft metrics of the
demodulated symbols by the weighting factors; and combining the
multiplied soft metrics.
6. The method of claim 5, wherein the LLR calculation step
comprises the steps of: calculating the LLRs of coded bits mapped
to a modulation symbol from each of the different modulation
schemes; calculating an average of the LLRs; and setting the
average LLR as the LLR for the different modulation schemes.
7. The method of claim 5, further comprising the steps of:
comparing a transmission gains of the modulation symbols from the
different modulation schemes; and updating the weighting factors
with the transmission gains, if the transmission gains are
different.
8. A method of combining demodulated symbols in a mobile
communication system including a transmitting apparatus that
modulates symbols in different modulation schemes and a receiving
apparatus that demodulates the modulation symbols in correspondence
with the different modulation schemes, the method comprising the
steps of: calculating a signal to noise ratio (SNR) of a modulation
symbol from each of the different modulation schemes; determining
relative ratios of the SNRs to be weighting factors for the
different modulation schemes; multiplying soft metrics of the
demodulated symbols by the weighting factors; and combining the
multiplied soft metrics.
9. The method of claim 8, further comprising the steps of:
comparing transmission gains of the modulation symbols from the
different modulation schemes; and resetting the weighting factors
with the transmission gains, if the transmission gains are
different.
10. The method of claim 9, wherein the transmission gains of the
modulation symbols are determined using numbers of Walsh codes used
for the modulation symbols.
11. A method of combining demodulated symbols in a mobile
communication system including a transmitting apparatus that
modulates symbols in different modulation schemes and a receiving
apparatus that demodulates the modulation symbols in correspondence
with the different modulation schemes, the method comprising the
steps of: calculating an energy of a modulation symbol from each of
the different modulation schemes; determining relative ratios of
the energies to be weighting factors for the different modulation
schemes; multiplying soft metrics of the demodulated symbols by the
weighting factors; and combining the multiplied soft metrics.
12. The method of claim 11, further comprising the steps of:
comparing transmission gains of the modulation symbols from the
different modulation schemes; and resetting the weighting factors
with the transmission gains, if the transmission gains are
different.
13. The method of claim 12, wherein the transmission gains of the
modulation symbols are determined using numbers of Walsh codes used
for the modulation symbols.
14. A method of combining demodulated symbols in a mobile
communication system including a transmitting apparatus that
modulates symbols in different modulation schemes and a receiving
apparatus that demodulates the modulation symbols in correspondence
with the different modulation schemes, the method comprising the
steps of: calculating an amplitude of a modulation symbol from each
of the different modulation schemes; determining relative ratios of
the amplitudes to be weighting factors for the different modulation
schemes; multiplying soft metrics of the demodulated symbols by the
weighting factors; and combining the multiplied soft metrics.
15. The method of claim 14, further comprising the steps of:
comparing transmission gains of the modulation symbols from the
different modulation schemes; and resetting the weighting factors
with the transmission gains, if the transmission gains are
different.
16. The method of claim 15, wherein the transmission gains of the
modulation symbols are determined using numbers of Walsh codes used
for the modulation symbols.
17. An apparatus for combining demodulated symbols in a mobile
communication system including a transmitting apparatus that
modulates symbols in different modulation schemes and a receiving
apparatus that demodulates the modulation symbols in correspondence
with the different modulation schemes, the apparatus comprising: a
reliability weighting factor calculator for calculating weighting
factors for the different modulation schemes; and a weighting
factor controller for multiplying the demodulated symbols by the
weighting factors.
18. The apparatus of claim 17, further comprising a soft symbol
controller for combining the multiplied values.
19. An apparatus for combining demodulated symbols in a mobile
communication system including a transmitting apparatus that
modulates symbols in different modulation schemes and a receiving
apparatus that demodulates the modulation symbols in correspondence
with the different modulation schemes, the apparatus comprising: a
weighting factor calculator for calculating a log likelihood ratio
(LLR) of a modulation symbol from each of the different modulation
schemes and determining relative ratios of the LLRs to be weighting
factors for the different modulation schemes; a weighting factor
controller for multiplying soft metrics of the demodulated symbols
by the weighting factors; and a soft symbol controller for
combining the multiplied soft metrics.
20. The apparatus of claim 19, wherein the weighting factor
calculator calculates the LLRs of coded bits mapped to the
modulation symbol from each of the different modulation schemes,
calculates an average of the LLRs, and sets the average LLR as the
LLR for the different modulation schemes.
21. The apparatus of claim 19, wherein the weighting factor
calculator compares transmission gains of modulation symbols from
the different modulation schemes, and resets the weighting factors
with the transmission gains, if the transmission gains are
different.
22. An apparatus for combining demodulated symbols in a mobile
communication system including a transmitting apparatus that
modulates symbols in different modulation schemes and a receiving
apparatus that demodulates the modulation symbols in correspondence
with the different modulation schemes, the apparatus comprising: a
weighting factor calculator for calculating a signal to noise ratio
(SNR) of a modulation symbol from each of the different modulation
schemes and determining relative ratios of the SNRs to be weighting
factors for the different modulation schemes; a weighting factor
controller for multiplying soft metrics of the demodulated symbols
by the weighting factors; and a soft symbol controller for
combining the multiplied soft metrics.
23. The apparatus of claim 22, wherein the weighting factor
calculator compares transmission gains of the modulation symbols
from the different modulation schemes, and resets the weighting
factors with the transmission, gains, if the transmission gains are
different.
24. The apparatus of claim 23, wherein the transmission gains of
the modulation symbols are determined using numbers of Walsh codes
used for the modulation symbols.
25. An apparatus for combining demodulated symbols in a mobile
communication system including a transmitting apparatus that
modulates symbols in different modulation schemes and a receiving
apparatus that demodulates the modulation symbols in correspondence
with the different modulation schemes, the apparatus comprising: a
weighting factor calculator for calculating an energy of a
modulation symbol from each of the different modulation schemes and
determining relative ratios of the energies to be weighting factors
for the different modulation schemes; a weighting factor controller
for multiplying soft metrics of the demodulated symbols by the
weighting factors; and a soft symbol controller for combining the
multiplied soft metrics.
26. The apparatus of claim 25, wherein the weighting factor
calculator compares transmission gains of modulation symbols from
the different modulation schemes, and updates the weighting factors
with the transmission gains, if the transmission gains are
different.
27. The apparatus of claim 26, wherein the transmission gains of
the modulation symbols are determined using numbers of Walsh codes
used for the modulation symbols.
28. An apparatus for combining demodulated symbols in a mobile
communication system including a transmitting apparatus that
modulates symbols in different modulation schemes and a receiving
apparatus that demodulates the modulation symbols in correspondence
with the different modulation schemes, the apparatus comprising: a
weighting factor calculator for calculating an amplitude of a
modulation symbol from each of the different modulation schemes,
and determining relative ratios of the amplitudes to be weighting
factors for the different modulation schemes; a weighting factor
controller for multiplying soft metrics of the demodulated symbols
by the weighting factors; and a soft symbol combiner for combining
the multiplied soft metrics.
29. The apparatus of claim 28, wherein the weighting factor
calculator compares transmission gains of modulation symbols from
the different modulation schemes, and resets the weighting factors
with the transmission gains, if the transmission gains are
different.
30. The apparatus of claim 29, wherein the transmission gains of
the modulation symbols are determined using numbers of Walsh codes
used for the modulation symbols.
Description
PRIORITY
[0001] This application claims priority to an application entitled
"Apparatus and Method for Symbol Combining in a Mobile
Communication System" filed in the Korean Industrial Property
Office on Nov. 19, 2001 and assigned Serial No. 2001-71832, the
contents of which are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates generally to a mobile
communication system using a retransmission scheme, and in
particular, to an apparatus and method for performing soft symbol
combining in a receiver.
[0004] 2. Description of the Related Art
[0005] In general, a receiver performs soft symbol combining to
improve reception performance in a mobile communication system
using a retransmission scheme (e.g., HARQ: Hybrid Automatic Repeat
reQuest).
[0006] FIG. 1 illustrates data retransmission using a different
modulation scheme and a different code rate for each transmission
in a communication system using a retransmission scheme. Referring
to FIG. 1, a transmitter transmits data using 16QAM (Quadrature
Amplitude Modulation) with a code rate (R) of 1/4 at a first
transmission. If a receiver fails to receive the data, it transmits
an NACK (Non-Acknowledgement) signal to the transmitter, requesting
retransmission of the data. The transmitter then transmits to the
receiver additional redundancy information using a different
modulation, QPSK (Quadrature Phase Shift Keying) with a code rate
of 1/2 at a second transmission (i.e., a first retransmission). The
receiver receives the redundancy information and subjects it to
soft symbol combining. If the retransmission scheme is CC (Chase
Combining), the transmitter transmits the same redundancy
information at the retransmission as at the initial transmission.
If retransmission scheme is IR (Incremental Redundancy), the
transmitter transmits the same or different redundancy information
at the retransmission.
[0007] Soft Symbol Combining is Performed in Two Ways.
[0008] (1) Homogeneous modulation scheme-based soft symbol
combining: when initial transmission and retransmission are carried
out using the same modulation scheme, the receiver combines soft
symbols output from a single demodulator irrespective of initial
transmission or retransmission. This soft symbol combining method
is adopted in an HARQ system using one modulation_scheme or an HARQ
system, which uses a plurality of modulation schemes but maintains
the same modulation scheme from an initial transmission of data to
the last retransmission of the data.
[0009] (2) Heterogeneous modulation scheme-based soft symbol
combining: when initial transmission and retransmission are carried
out using different modulation schemes, the receiver combines soft
symbols output from different demodulators at the initial
transmission and retransmission. This soft symbol combining method
is adopted in a communication system using an AMCS (Adaptive
Modulation and Coding Scheme) and an adaptive HARQ.
[0010] The homogeneous modulation scheme and the heterogeneous
modulation scheme will be described below in more detail.
[0011] FIGS. 2 and 3 are block diagrams of conventional receivers
operating according to the homogeneous modulation scheme and
according to the heterogeneous modulation scheme, respectively. It
is assumed here that a transmitter selects one modulation scheme in
an HARQ algorithm at each data transmission. The operation of the
transmitter depends on system implementation, and thus its
description is not provided here.
[0012] Referring to FIGS. 2 and 3, the receivers obtain soft
metrics (soft outputs) from demodulators 201-1 and 202-1
(hereinafter, collectively referred to as 201) and demodulators
202-1 and 202-2 (hereinafter, collectively referred to as 202) that
operate according to different modulation schemes used for initial
transmission and retransmission. Soft symbol controllers 203-1 and
203-2 (hereinafter, collectively referred to as 203) output the
arithmetic sum of the soft metrics as a combined soft symbol metric
(output) when soft symbol combining is used. The soft symbol
controllers 203 rearrange the soft symbols in the original order
when soft symbol combining is not used. Though not illustrated,
normalization blocks may be disposed between the demodulators 201
and 202 and turbo decoders 204-1 and 204-2 (hereinafter,
collectively referred to as 204) in order to prevent overflow
caused by fixed-point operation. Normalization can be performed in
many well-known methods and thus it will not be described here.
[0013] Because the receiver illustrated in FIG. 2 operates
according to the homogeneous modulation scheme, the demodulators
201-1 and 202-1 support the same modulation scheme, QPSK. On the
other hand, since the receiver illustrated in FIG. 3 operates
according to the heterogeneous modulation scheme, the demodulators
201-2 and 202-2 support different modulation schemes, QPSK and
16QAM, respectively. The demodulators 201 demodulate initial
transmission data and the demodulators 202 demodulate
retransmission data. When the soft symbol controllers 203 start to
operate, the outputs of the demodulators 201 and 202 are provided
simultaneously. While the receiver supports two modulation schemes
in FIG. 3 for illustrative purposes, it can be expanded to support
more modulation schemes by using more demodulators.
[0014] The demodulators illustrated in FIGS. 2 and 3 can output
soft metrics in many ways (e.g., by DMM (dual minimum metric)
provided in "Evaluation Methodology", a system simulation guidebook
presented by the 3GPP2 (3.sup.rd Generation Partnership Project 2),
or by maximum likelihood metric to minimize errors). The present
invention can be implemented with use of any soft metric generation
method. Here, DMM will be adopted by way of example.
[0015] In the conventional soft symbol combining method as
illustrated in FIGS. 2 and 3, irrespective of modulation schemes
used for an initial transmission and retransmissions, a soft metric
is obtained independently according to a corresponding modulation
scheme for each transmission. Then the soft symbol controllers 203
operate differently, depending on whether soft symbol combining is
used or not. Specifically, the two demodulators 201 and 202
directly feed soft metrics to the soft symbol controllers 203. If
soft symbol combining is used, the soft symbol controllers 203
arithmetically calculate the average of the two soft metrics. If
soft symbol combining is not used, the soft symbol controllers 203
rearrange soft symbols.
[0016] In view of the nature of turbo codes and the performance of
a MAP (Maximum A Posteriori) decoder, a LogMAP decoder, and a
MaxLogMAP decoder used as the turbo decoders 204, the following
considerations must be taken into account in the above direct soft
metric feeding from demodulators using different modulation schemes
to a turbo decoder.
[0017] In general, a channel reliability L.sub.c is multiplied with
an input soft metric during turbo decoding in a turbo decoder 204
illustrated in FIG. 2 or 3. In an AWGN (Additive White Gaussian
Noise) environment, the channel reliability
L.sub.c=4E.sub.b/N.sub.o and increases proportionally to the SNR
(Signal-to-Noise Ratio) of the channel. Thus, representing the
reliability of the soft symbol, the channel reliability L.sub.c is
a kind of weighting factor that varies with the quality of the soft
symbol when the turbo decoder 204 performs MAP decoding. If the SNR
changes, therefore, this weighting factor is changed. Because the
target SNR of the AWGN channel is preset, the channel reliability
L.sub.c is determined according to the target SNR.
[0018] The channel reliability L.sub.c is a very significant factor
to the MAP decoder. It is well known that an estimation error
(e.g., overestimation or underestimation) of the channel
reliability L.sub.c greatly degrades decoding performance. To
minimize the performance degradation, a MaxLogMAP decoder is used
instead of the MAP decoder.
[0019] If a signal processed by the homogenous modulation scheme is
transmitted on a static channel such as AWGN, there is no change in
the channel reliability L.sub.c. Therefore, the soft symbol
controller 203-1 simply calculates the average of soft metrics from
the demodulators 201-1 and 202-1 in the receiver illustrated in
FIG. 2 because the channel reliability of the soft metrics are
identical.
[0020] In the heterogeneous modulation scheme, however, the channel
reliability L, is different for different modulation schemes. For
example, if a QPSK soft metric is combined with an 8PSK or 16QAM
soft metric, the modulation schemes have different channel
reliabilities. In order to optimize decoding performance, the soft
metrics must be weighted according to the channel reliabilities of
the modulation schemes for the same reason that the channel
reliabilities L.sub.c are weighted according to channel condition
in a turbo decoder as stated above.
[0021] When a 16QAM soft metric is combined with a QPSK soft metric
in actual implementation, weighting the soft metrics at a ratio of
1:3 (16QAM:QPSK) produces an about 0.8 dB gain increase which might
not otherwise be obtained. Here, combining covers both soft symbol
combining and symbol rearrangement. Consequently, soft metrics must
be weighted according to channel reliability when they are from
heterogeneous demodulators. In fact, the conventional symbol
combining method focuses mainly on symbol combining of homogenous
symbols. Even when it deals with symbol combining of heterogeneous
symbols, the same channel reliability L.sub.c is simply applied
without weighting during turbo decoding. As a result, decoding
performance is degraded.
SUMMARY OF THE INVENTION
[0022] It is, therefore, an object of the present invention to
provide an apparatus and method in a receiver for assigning
reliability weight factors (RWFs) to soft metrics output from
demodulators in a mobile communication system using different
modulation schemes.
[0023] To achieve the above and other objects, an apparatus and
method for soft- combining demodulated symbols in a mobile
communication system are provided. In the mobile communication
system, a transmitting apparatus modulates symbols in different
modulation schemes and a receiving apparatus demodulates the
modulation symbols in correspondence with the modulation schemes.
The soft symbol combining apparatus includes an RWF calculator, an
RWF controller, and a soft symbol controller.
[0024] According to one aspect of the present invention, the RWF
calculator calculates RWFs for the modulation schemes. The RWF
controller multiplies the demodulated soft symbols by the RWFs. The
soft symbol controller combines the multiplied values.
[0025] According to another aspect of the present invention, the
RWF calculator calculates an LLR (Log Likelihood Ratio) of a
modulation symbol from each of the modulation schemes and
determines relative ratios of the LLRs to be RWFs for the
modulation schemes. The RWF controller multiplies the soft metrics
of the demodulated symbols by the RWFs. The soft symbol controller
combines the multiplied soft metrics.
[0026] According to a further aspect of the present invention, the
RWF calculator calculates an SNR of a modulation symbol from each
of the modulation schemes and determines relative ratios of the
SNRs to be RWFs for the modulation schemes. The RWF controller
multiplies the soft metrics of the demodulated symbols by the RWFs.
The soft symbol controller combines the multiplied soft
metrics.
[0027] According to still another aspect of the present invention,
the RWF calculator calculates an energy of a modulation symbol from
each of the modulation schemes and determines relative ratios of
the energies to be RWFs for the modulation schemes. The RWF
controller multiplies the soft metrics of the demodulated symbols
by the RWFs. The soft symbol controller combines the multiplied
soft metrics.
[0028] According to yet another aspect of the present invention,
the RWF calculator calculates an amplitude of a modulation symbol
from each of the modulation schemes and determines relative ratios
of the amplitudes to be RWFs for the modulation schemes. The RWF
controller multiplies the soft metrics of the demodulated symbols
by the RWFs. The soft symbol combiner combines the multiplied soft
metrics.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] The above and other objects, features, and advantages of the
present invention will become more apparent from the following
detailed description when taken in conjunction with the
accompanying drawings in which:
[0030] FIG. 1 illustrates data retransmission using a different
modulation and a different code rate for each transmission in a
communication system using a retransmission scheme;
[0031] FIG. 2 is a block diagram of a conventional receiver that
operates according to a homogeneous modulation scheme;
[0032] FIG. 3 is a block diagram of a conventional receiver that
operates according to a heterogeneous modulation scheme;
[0033] FIG. 4 is a block diagram of a transmitting apparatus and a
receiving apparatus using a plurality of modulation schemes, to
which the present invention is applied;
[0034] FIG. 5 is a block diagram of an apparatus for combining soft
metrics from a plurality of demodulators in a heterogeneous
modulation-based system according to an embodiment of the present
invention;
[0035] FIG. 6 illustrates a reliability weighting factor (RWF)
controller illustrated in FIG. 5;
[0036] FIG. 7 illustrates an 8PSK signal constellation;
[0037] FIG. 8 illustrates a 16QAM signal constellation;
[0038] FIG. 9 illustrates graphs comparing the performances of
homogeneous modulation and heterogeneous modulation according to
whether RWF is applied or not; and
[0039] FIG. 10 is a flowchart illustrating an operation in a
receiver for calculating RWFs for the soft metrics of demodulated
symbols in a heterogeneous modulation-based system according to the
embodiment of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0040] Preferred embodiments of the present invention will be
described herein below with reference to the accompanying drawings.
In the following description, well-known functions or constructions
are not described in detail since they would obscure the invention
in unnecessary detail.
[0041] The following description is made with the appreciation that
the present invention is not limited to HARQ, but pertains to soft
symbol combining or symbol rearrangement of demodulator outputs
irrespective of whether the inputs of demodulators are all initial
transmission data, retransmission data, or both initial
transmission and retransmission data, and whether the demodulators
demodulate simultaneously or sequentially. Terms "homogeneous" and
"heterogeneous" are used herein to indicate use of the same
modulation scheme and use of different modulation schemes,
respectively.
[0042] FIG. 4 is a block diagram of a transmitting apparatus and a
receiving apparatus using a plurality of modulation schemes, to
which the present invention is applied. Referring to FIG. 4, in the
transmitting apparatus, a channel encoder 401 generates forward
error correction codes (FECs) to correct errors in a channel 407. A
redundancy selector 402 selects redundancy information according to
a code rate in a given redundancy selection method, upon request
for data retransmission. CC (Chase Combining) and IR (Incremental
Redundancy) are implemented usually by use of the redundancy
selector 402. A multiplexer (MUX) 403 outputs the redundancy
information to a modulator using a modulation scheme corresponding
to the current transmission among a plurality of modulators 404-1
to 404-N. The modulator modulates the redundancy information in its
modulation scheme. A spreader 405 spreads the modulated symbol with
a predetermined spreading code if the system is a CDMA (Code
Division Multiple Access) system. A transmitter 406 upconverts the
spread signal to an RF (Radio Frequency) signal suitable for
transmission and transmits the RF signal on the channel 407.
[0043] The receiving apparatus operates in the reverse order of the
transmitting apparatus. A receiver 408 downconverts an RF signal
received on the channel 407 to a baseband signal. A despreader 409
despreads the baseband signal with a predetermined spreading code.
Demodulators 410-1 to 410-N demodulate the spread signal according
to their demodulation schemes. A demultiplexer (DEMUX) 411
selectively feeds the outputs of the demodulators 410-1 to 410-N to
a soft symbol controller 412. The soft symbol controller 412
combines soft metrics received from the DEMUX 411 when soft symbol
combining is used, and rearranges code symbols (i.e., redundancy
information) in the original order when soft symbol combining is
not used. A channel decoder 413 decodes the output of the soft
symbol combiner 412. A modulation & coding controller 414
commonly controls the transmitting and receiving apparatuses. The
modulation & coding controller 414 selects a code rate,
redundancy information, and a modulation scheme and correspondingly
controls the redundancy selector 402, the soft symbol controller
412, the MUX 403, and the DEMUX 411. to the criteria for making the
selection is dependent on system implementation and thus its
detailed description is not provided here. The soft symbol
controller 412, which is the main feature of the present invention,
will be described later in more detail.
[0044] FIG. 5 is a block diagram of an apparatus for combining soft
metrics output from a plurality of demodulators in a heterogeneous
modulation-based system according to an embodiment of the present
invention. Referring to FIG. 5, the demodulators 410-1 to 410-N
demodulate a signal received from the despreader 409 illustrated in
FIG. 4 in different demodulation schemes. For example, the
demodulator 410-2 demodulates in QPSK, and the demodulator 410-N
demodulates in 16QAM. An RWF calculator 503 calculates an RWF to be
assigned to a soft metric from each demodulator. The RWF is a
constant fixed for the demodulator for as long as the demodulator
uses the same modulation scheme. An RWF controller 501 multiplies
soft metrics from the demodulators 410-1 to 410-N by RWFs assigned
to the respective soft metrics. The soft symbol controller 412
combines a plurality of soft metrics received from the RWF
controller 501 and feeds the combined soft metric to the channel
decoder 413. Here, a turbo decoder is used as the channel decoder
413.
[0045] According to the embodiment of the present invention, when
different modulation schemes are used, the receiving apparatus is
provided with the RWF controller 501 to control a relative channel
reliability L, for each demodulator. The RWF controller 501
functions to reflect channel reliabilities in the soft metrics
received from the demodulators 410-1 to 410-N to achieve optimum
decoding performance in the turbo decoder 413, after soft symbol
combining in the soft symbol controller 412. In other words, the
RWF controller 501 controls weighting factors. The weighting
factors are determined according to the modulation schemes and the
SNRs of modulation symbols.
[0046] Methods of obtaining RWFs will be described herein
below.
[0047] As illustrated in FIG. 5, calculation of RWFs is most
significant in weighting soft metrics from demodulators in a
receiver that operates according to a heterogeneous modulation
scheme. The RWFs for soft metrics from the demodulators are
calculated such that demodulated symbols corresponding to different
modulation schemes have equal reliability prior to turbo decoding.
The RWF calculator 503 performs the RWF calculation. RWF
calculation can be implemented in soft ware, or in hard ware (e.g.,
a look-up table (LUT) or a circuit). When RWFs are calculated
utilizing soft ware, a program for invoking an RWF calculation
formula may be stored in a ROM (Read Only Memory). The RWF
controller 501 multiplies the RWFs by corresponding soft metrics.
The RWF controller 501 can also be implemented in hardware (e.g., a
calculator) or in soft ware (e.g., a program).
[0048] FIG. 6 illustrates an embodiment of the RWF controller 501
according to the present invention. Referring to FIG. 6, the RWF
controller 501 includes as many multipliers 601-1 to 601-N as the
demodulators 410-1 to 410-N illustrated in FIG. 5. The multiplier
601-1 multiplies a soft metric received from the first demodulator
410-1 by an RWF (RWF 1) received from the RWF calculator 503 and
the multiplier 601-2 multiplies a soft metric received from the
second demodulator 410-2 by an RWF (RWF 2) received from the RWF
calculator 503. In the same manner, the multiplier 601-N multiplies
a soft metric received from the Nth demodulator 410-N by an RWF
(RWF N) received from the RWF calculator 503. That is, the RWF
controller 501 multiplies soft metrics from the demodulators 410-1
to 410-N by RWFs assigned to the respective soft metrics.
[0049] As stated above, in the present invention, it is preferable
to weight a 16QAM soft metric and a QPSK soft metric at a ratio of
1:3. Also, the weighting ratio is similar to the transmitted signal
amplitude ratio 1:3.16 (={square root}{square root over (10)}:1) of
the modulation schemes. This implies that the SNRs of modulation
symbols from different modulation schemes are closely related to
their channel reliabilities L.sub.c. Related simulation results
will be described below.
[0050] If an initial transmission and a retransmission are carried
out according to a heterogeneous modulation scheme, application of
a common RWF to heterogeneous soft metrics without considering
different modulations leads to performance degradation during turbo
decoding, as described above. A simulation using the same number of
Walsh codes at the initial transmission and retransmission has
revealed that weighting an 8PSK soft metric and a QPSK soft metric
with an RWF of 1/3 and an RWF of 1/4, respectively, results in the
best performance in turbo decoding. Table 1 and Table 2 illustrate
the results of this simulation.
1TABLE 1 RWF BWR PER 2 2.42e-03 3.40e-02 2.8 1.90e-03 2.40e-02 2.9
1.60e-03 2.10e-02 2.95 1.45e-03 1.90e-02 3 1.46e-03 1.90e-02 3.1
1.66e-03 2.40e-02 3.16 1.80e-03 2.50e-02 4 3.43e-03 3.50e-02
[0051]
2TABLE 2 RWF BWR PER 2 4.21e-03 2.40e-02 3 4.15e-03 1.70e-02 3.8
5.44e-03 1.50e-02 3.9 8.28e-03 1.20e-02 3.95 8.01e-03 1.20e-02 3.97
7.77e-03 1.20e-02 4 1.05e-03 1.50e-02 4.2 1.10e-03 1.80e-02
[0052] Referring to Table 1, when an initial transmission is
carried out in 16QAM and a retransmission is carried out in QPSK,
with the same transmission power, when an RWF for QPSK with respect
to 16QAM is 3 or its approximate value (e.g., 2.95 or 3), the
lowest PER (Packet Error Rate), 1.90e-02 is obtained.
[0053] Referring to Table 2, when an initial transmission is
carried out in 8PSK and a retransmission is carried out in QPSK,
with the same transmission power, when an RWF for QPSK with respect
to 8PSK is an approximate value to 4 (e.g., 3.9, 3.95, or 3.97),
the lowest PER, 1.20e-02 is obtained.
[0054] Taking the simulation results into account, the following
RWF calculation methods are proposed. They can be selectively used
according to implementation complexity and performance. A combined
use of the RWF calculation methods is available depending on a
system. A detailed description of new RWF calculation methods
produced by combining the proposed ones is not given here.
[0055] Method 1
[0056] In a first method, RWFs are calculated using a ratio of the
SNRs, energies, or amplitudes of modulation symbols from different
modulation schemes. For example, if the SNR of a QPSK modulation
symbol is A and that of a 16QAM modulation symbol is B, their RWF
ratio is A:B. Here, A and B are linear.
[0057] Also, RWFs can be calculated according to the amplitudes of
modulation symbols, for example, by Eq. (1) below. In Eq. (1), the
numerator represents the amplitude of a modulation symbol from a
modulation scheme A. The denominator represents the amplitude of a
modulation symbol from a modulation scheme B.
[0058] Alternatively, RWFs can be calculated based on the energies
of modulation symbols, for example, by Eq. (2) below. In Eq. (2),
the numerator represents the energy of a modulation symbol from the
modulation scheme A. The denominator represents the amplitude of a
modulation symbol from the modulation scheme B. 1 RWF = [ 1 L k L (
X k 2 + Y k 2 ) Modulation A 1 L k L ( X k 2 + Y k 2 ) Modulation B
] ( 1 ) RWF = [ 1 L k L ( X k 2 + Y k 2 ) Modulation A 1 L k L ( X
k 2 + Y k 2 ) Modulation B ] ( 2 )
[0059] The above equation 1 represents a theoretical process that
amplitudes of L modulation symbols are respectively calculated
based on the modulation scheme A and then the amplitudes of L
modulation symbols are also calculated based on the modulation
scheme B, thereby calculating RWFs, wherein a variable L denotes
the number of modulation symbols to be measured for the purpose of
comparison to the amplitude, and variables X.sub.k and Y.sub.k
denote an in-phase component value and a quadrature-phase component
value of a k-th modulation symbol, respectively. For example, a
phase component value of a modulation symbol S.sub.k corresponding
to a X-axis defined as I-Channel in FIG. 7 is represented as
X.sub.k, while a phase component value of the modulation symbol
S.sub.k corresponding to a Y-axis defined as Q-Channel is
represented as Y.sub.k. That is, one modulation symbol S.sub.k may
be represented in the form of (X.sub.k, Y.sub.k) on the orthogonal
coordinate. Thus, as seen in the equation 1, it should be noted
that a series of operations, i.e., squaring X.sub.k and Y.sub.k,
adding the squared values to each other and then calculating a
square root with respect to the added result is identical with a
process of calculating a distance from the center to a location of
a given symbol using the orthogonal coordinate.
[0060] In equation 2, variables L, X.sub.k and Y.sub.k are
substantially same as those defined in equation 1. However, the
equation 2 indicates an alternative process, quite different from
that of the equation 1 that energies of the modulation symbols are
calculated by each modulation scheme and then RWFs are calculated
therefrom. According to this process, instead of calculating a
square root with respect to a squared sum of X.sub.k and Y.sub.k,
the squared results are added to each other, so that an average
energy for respective modulation schemes is calculated to be used
as a factor for calculating the RWFs.
[0061] Method 2
[0062] In a second method, RWFs are calculated using an average LLR
(Log Likelihood Ratio) or soft metric ratio of a plurality of code
symbols mapped to a modulation symbol from each modulation scheme.
In QPSK, two code symbols s0 and s1 are mapped to one modulation
symbol and the average LLR, Avg_LLR (QPSK), of the two code symbols
s0 and s1 is calculated. In 8PSK, three code symbols s0, s1, and s2
are mapped to one modulation symbol as illustrated in FIG. 7 and
the average LLR, Avg_LLR (8PSK), of the three code symbols s0, s1,
and s2 is calculated. In 16QAM, four code symbols s0, s1, s2 and s3
are mapped to one modulation symbol and the average LLR, Avg_LLR
(16QAM), of the four code symbols s0, s1, s2, and s3 is calculated.
In the same manner, the average LLR, Avg_LLR (64QAM), of code
symbols mapped to a 64QAM modulation symbol is calculated. Ratios
of the calculated average LLRs are used as RWFs. This is expressed
as 2 RWF = 1 LM k = 1 L i = 0 m - 1 Soft_Metric ( k , i ) = 1 LM k
= 1 L i = 0 m - 1 K log ( Pr { s k , i = 1 | X k , Y k } Pr { s k ,
i = 0 | X k , Y k } ) ( 3 )
[0063] The above equation 3 represents a process that a Soft_Metric
(k, i) with respect to L number of modulation symbols is
calculated, and then RWFs are calculated, wherein a variable L
denotes the number of the modulation symbols to be measured for the
purpose of comparison to the amplitude size, and a variable M
denotes a code bit constituting one symbols. For example, M is 3 in
FIG. 7. That is, one modulation symbol S.sub.k consists of three
code symbols, Sk.sub.0, S.sub.k1, S.sub.k2. In the equation 3,
X.sub.k and Y.sub.k denotes an in-phase component value and a
quadrature-phase component value of the k-th modulation symbol,
respectively. For example, a phase component value of a modulation
symbol S.sub.k corresponding to a X-axis defined as I-Channel in
FIG. 7 is represented as X.sub.k, and a phase component value of a
modulation symbol S.sub.k corresponding to a Y-axis defined as
Q-Channel is represented as Y.sub.k. That is, one symbol S.sub.k
may be represented inf the form of (X.sub.k, Y.sub.k) using the
orthogonal coordinate. Also, in the equation 3, Pr{A.vertline.B,C}
represents a probability of the occurrence of an event A under a
condition that both an event B and an event C occur. K is a
constant for normalization. Factors, i and k represents an order of
the code symbols and modulation symbols, respectively. The equation
3 represents a process of calculating a probability that respective
code symbols of the kth modulation symbols, X.sub.k and Y.sub.k
become "0" or "1", so that a soft matric is calculated from the
calculated probability value and then the RWFs are calculated from
an average value of the soft metric.
[0064] Method 3
[0065] In a third method, RWFs are calculated by simplifying or
approximating Eq. (3). For example, Eq. (3) is wholly approximated
or only Soft_Metric (k, i) is approximated. The approximation of
Eq. (3) will be described later.
[0066] Method 4
[0067] Due to the complexity of Eq. (3), RWFs resulting from a
simulation (e.g., Table 1 and Table 2) are used consistently. This
method will be described later in more detail.
[0068] FIGS. 7 and 8 illustrate signal constellations for 8PSK and
16QAM, respectively. As illustrated, m coded bits are mapped to a
modulation symbol in each modulation scheme. As in Eq. (3), the
soft metric or LLR .LAMBDA.(S.sub.k,1) of a code bit S.sub.k,1
(i=0, 1, . . . , m-1) is calculated by 3 ( s k , i ) = K log Pr { s
k , i = 1 | X k , Y k } Pr { s k , i = 0 | X k , Y k } , i = 0 , 1
, , m - 1 ( 4 )
[0069] where K is a constant and Pr{A.vertline.B} is a conditional
probability defined as the probability of the occurrence of an
event A when an event B occurs. X.sub.k and Y.sub.k denote the
X-axis (I channel) and Y-axis (Q channel) coordinates of a received
symbol in FIGS. 7 and 8. According to communication theory, X.sub.k
and Y.sub.k represent the in-phase and quadrature-phase signal
components of the symbol, respectively. Since Eq. (4) exhibits
non-linear characteristics and requires a relatively large volume
of computation, a simple algorithm for approximating Eq. (4) is
usually adopted in actual implementation. Soft metric calculation
using Eq. (4) is well known and thus its description is not
provided here. Yet, for better understanding of the present
invention, methods of approximating Eq. (4) will be described.
[0070] When an M-ary QAM and an 8PSK are adopted, one of the
approximation methods is DMM. Even for an M-ary QAM and an M-ary
PSK that have relatively high modulation orders, soft metrics can
be obtained using Eq. (4).
[0071] In 16QAM, for example, the soft metrics or LLRs of code bits
as calculated by Eq. (4) are approximated by Eq. (5). because Eq.
(5) is valid only in an AWGN channel environment, appropriate
compensation is required in a fading channel environment where the
SNR of a received signal varies moment to moment.
.LAMBDA.(s.sub.k,1)=X.sub.k
.LAMBDA.(s.sub.k,0)=.vertline.X.sub.k.vertline.-2a
.LAMBDA.(s.sub.k,3)=Y.sub.k
.LAMBDA.(s.sub.k,2)=.vertline.Y.sub.k.vertline.-2a (5)
[0072] where, "a" means a distance from the center on the
orthogonal coordinate.
[0073] In 8PSK, the soft metrics or LLRs of code bits as calculated
by Eq. (4) is approximated by 4 ( s k , 2 ) = Y k ( s k , 1 ) = X k
( s k , 0 ) = 1 - | Y k X k | ( 6 )
[0074] As noted from Eq. (5) and Eq. (6), although coded bits are
from the same received symbol (X.sub.k, Y.sub.k), they have
different soft metrics. "The same received symbol" is used in the
sense of the same modulation symbol energy or the same modulation
symbol SNR. Thus, it can be concluded that the soft metrics of
coded bits are different according to modulation schemes even if
they are from the same received symbol.
[0075] Eq. (5) and Eq. (6) can be used for approximation in Method
3. In accordance with the embodiment of the present invention
including Method 1, Method 2, and Method 3, if one modulation
scheme is used-and the SNRs of modulations symbols are equal, a
predetermined RWF is applied to the soft metric of each of the code
bits from the modulation symbols. For example, in 8PSK, an RWF of
4E.sub.b/N.sub.o (=L,) is applied. On the other hand, if different
modulation schemes are used, RWFs for heterogeneous symbols are
calculated and heterogeneous soft metrics are weighted with the
RWFs in the following steps.
[0076] Step 1: an average soft metric is calculated for each
modulation scheme;
[0077] Step 2: a relative ratio of the average soft metrics is
calculated; and
[0078] Step 3: the relative soft metric ratios are applied as RWFs
to soft metrics output from demodulators.
[0079] While optimum RWFs can be achieved by developing equations
from the above steps, the equation development requires high
accuracy and is very complicated. In addition, errors may be
involved with approximation of solutions to the equations.
[0080] It can be further contemplated as another embodiment that
instead of Step 1, the average of soft metrics output from
demodulators corresponding to different modulation schemes is
calculated on the assumption that modulation symbols from the
different modulation schemes are transmitted with the same power in
an AWGN channel environment with the same channel noise. The second
embodiment of the present invention corresponds to Method 4. This
method is advantageously accurate when errors are generated in
equations because RWFs are obtained from a simulation as
illustrated in Table 1 and Table 2.
[0081] FIG. 9 illustrates graphs showing a performance comparison
between a homogeneous modulation scheme and a heterogeneous
modulation scheme according to whether RWFs are used or not. The
performance is presented in spectrum efficiency with respect to
E.sub.c/N.sub.t when signals are weighted with RWFs.
[0082] Referring to FIG. 9, solid lines indicate simulation results
when RWFs are used, and dotted lines indicate simulation results
when RWFs are not used. Lines marked with indicate signal
transmission in 16QAM with R=3/8 at initial transmission and
retransmission, while lines marked with X indicate signal
transmission in 16QAM with R=3/8 at initial transmission and in
QPSK with R=3/4 at retransmission. As noted, there is no
performance difference between the initial transmission and the
retransmission when the same modulation scheme is used. That is,
when 16QAM is adopted at the initial transmission and the
retransmission, a negligibly slight performance difference is
observed even if RWFs are used (Diff_Weight). However, if different
modulation schemes are used at the initial transmission and the
retransmission, a very large performance difference results
depending on whether RWFs are used or not. Given the same spectral
efficiency, the performance difference between using RWFs
(Diff_Weight) and not using RWFs (Even_Weight) is about 1.0 dB or
greater in E.sub.c/N.sub.t. Accordingly, RWFs must be applied to
initial transmission symbols and retransmission symbols when
different modulation schemes are used.
[0083] Another consideration to be taken into account in using RWFs
is that the RWFs vary according to a ratio of the numbers of
available Walsh codes when different modulation schemes are used.
For example, if 16QAM and QPSK are used at an initial transmission
and a retransmission, respectively, as the ratio of the number of
Walsh codes used for the retransmission to that of Walsh codes used
for the initial transmission increases, an RWF assigned to 16QAM
decreases, in order to achieve optimum performance. This is because
the SNR of code symbols increases in proportion to the number of
Walsh codes under the same channel environment, that is, when the
same noise power is given. Thus, a channel reliability needs to be
increased in proportion to the SNR for the same reason that an RWF
is determined according to the SNR of a modulation symbol. In other
words, if different symbol energy is assigned to each transmission
scheme, the symbol energy difference needs to be reflected in RWFs.
First of all, RWFs are calculated in Method 1, Method 2, or Method
3. An energy ratio for each modulation scheme is then reflected in
the RWFs. Here, information about the number of Walsh codes used
for each transmission is delivered to the receiving apparatus on a
preset message channel or by signaling.
[0084] FIG. 10 is a flowchart illustrating an operation in a
receiving apparatus for calculating RWFs to be assigned to the soft
metrics of modulation symbols in the heterogeneous modulation-based
system according to the embodiment of the present invention. RWFs
are obtained in an optimal method based on LLRs or in a sub-optimal
method based on SNR, energy or amplitude. This procedure is
performed in the RWF calculator 503.
[0085] Referring to FIG. 3, the RWF calculator 503 determines
whether different modulation schemes are used in step 1001. If
different modulation schemes are used, i.e., a heterogeneous
modulation scheme, the procedure goes to step 1003. On the other
hand, for a homogeneous modulation scheme, which implies that a
plurality of demodulators use the same modulation scheme, the RWF
calculator 503 determines an RWF to be commonly assigned to soft
metrics from the demodulators and feeds the RWF to the RWF
controller 501 in step 1021.
[0086] In step 1003, the RWF calculator 503 starts to calculate
RWFs using modulation symbols (X.sub.k, Y.sub.k) from N modulation
schemes. A modulation symbol is represented by M code bits
(s.sub.0, s.sub.1, . . . , s.sub.M-1). The RWF calculator 503
calculates the LLRs (LLR(i).sub.0, LLR(i).sub.1, . . . ,
LLR(i).sub.M-1) of M code bits (s.sub.0, s.sub.1, . . . ,
s.sub.M-1) in a modulation symbol from each modulation scheme in
step 1005. Needless to say, modulation symbols are randomly
selected to thereby obtain average modulation symbols for the
respective modulation schemes. The LLRs may be computed
arithmetically, or statistically by simulation.
[0087] The RWF calculator 503 calculates the average of the M LLRs
for each modulation scheme, Avg_LLR(1), Avg_LLR(2), Avg_LLR(3), . .
. , Avg_LLR(N) in step 1007. In step 1009, the RWF calculator 503
calculates a relative ratio of the N average LLRs. For example, if
the average LLR for QPSK is Avg_LLR(1), relative ratios of the
other average LLRs to the average LLR for QPSK
(Avg_LLR(i)/Avg_LLR(QPSK) are calculated to 1:H.sub.1:H.sub.2: . .
. :H.sub.N, and are set as RWFs. While it has been described that
the RWFs are calculated, RWFs mapped to combinations of modulation
schemes can be preset and stored in a memory.
[0088] In step 1011, the RWF calculator 503 compares the final
transmission gains of the N modulation schemes. The final
transmission gain is influenced by the number of Walsh codes. In
other words, the final transmission gain can be estimated using the
number of Walsh codes used for transmission. The receiving
apparatus receives information about the number of Walsh codes used
for each transmission from the transmitting apparatus (usually a
base station) on a message channel or by signaling. If the final
transmission gains are equal, the RWF calculator 503 feeds the RWFs
to the RWF controller 412 in step 1013. If the final transmission
gains are different, the RWF calculator 503 multiplies the linear
value of the relative transmission gain of each modulation scheme
by a corresponding RWF and determines the product to be a new RWF
for the modulation scheme in step 1015. The above LLR-based RWF
calculation is an optimal method.
[0089] Now a sub-optimal RWF calculating method based on SNR,
energy, or amplitude will be described.
[0090] The RWF calculator 503 starts to calculate RWFs using
modulation symbols (X.sub.k, Y.sub.k) from the N modulation schemes
in step 1003. In step 1007, the RWF calculator 503 calculates the
average SNR, energy or amplitude of the modulation symbols
(X.sub.k, Y.sub.k). The RWF calculator 503 calculates relative
ratios of the average values, 1:H.sub.1:H.sub.2: . . . :H.sub.N,
and sets the relative ratios as RWFs in step 1019. Then the
procedure goes to step 1011.
[0091] In the embodiment of the present invention, the average LLR
of a modulation symbol from each modulation scheme, or the average
SNR, energy or amplitude of the modulation symbol is calculated.
Then RWFs are obtained from a relative ratio of the average values.
If each modulation scheme has a different transmission gain for
transmission symbols, the RWFs are updated by multiplying the RWFs
by transmission gains.
[0092] In accordance with the present invention as described above,
soft metrics output from a plurality of demodulators are weighted
with RWFs in soft symbol combining in a receiver in a heterogeneous
modulation-based system. Therefore, the decoding performance of a
channel decoder after the soft symbol combining is optimized.
[0093] While the invention has been shown and described with
reference to a certain preferred embodiment thereof, it will be
understood by those skilled in the art that various changes in form
and details may be made therein without departing from the spirit
and scope of the invention as defined by the appended claims.
* * * * *